1. Field of the Invention
The present invention relates in general to emission control equipment for fossil fuel fired power plants, and in particular, to a new and useful method and apparatus for SO.sub.x, NO.sub.x, and particulate control.
2. Description of the Related Art
Current energy policy in the U.S. is based on expanded use of coal in utility and industrial applications. This must not compromise environmental requirements however. Advanced control technologies are needed to control the increase in pollutant emissions of coal combustion. These pollutants include particulates, sulfur oxides (SO.sub.x), and oxides of nitrogen (NO.sub.x).
Fly ash and other particulate material can effectively be controlled using baghouses or fabric filterhouses. U.S. Pat. No. 4,309,386 which is assigned to the assignee of the present invention, discloses a hot catalytic baghouse which simultaneously removes particulate material and reduces the NO.sub.x emissions. This patent describes coating the catalyst onto the fabric of the baghouse filter media including the use of a fabric filter in which the catalyst is woven into the fabric.
U.S. Pat. No. 4,793,981 also assigned to the assignee of the present application describes the use of a hot catalytic baghouse which simultaneously collects sulfur oxides, nitrogen oxides and particulates.
Selective catalytic reduction (SCR) of NO.sub.x by ammonia is a well-established art. Various catalyst types and shapes are utilized in stand-alone reactors, whereby flue gas is mixed with small quantities of ammonia and then contacted with the catalyst. Also, sorbent or alkali injection into the boiler and the ducts is a common approach for sulfur dioxide (SO.sub.2) control from flue gas. The location of the sorbent injection may vary depending upon the type of sorbent.
U.S. Pat. No. 4,602,673 discloses an apparatus for preheating combustion air while simultaneously reducing NO.sub.x contained in the flue gas. By combining a catalytic reactor with an air heater, a compact device is possible according to that patent. However, the catalyst will have to be replaced frequently because of erosion of the catalyst due to dust build-up. Another major problem not addresssed by this patent is poisoning of the catalyst from SO.sub.x in the flue gas. In addition, fly ash erosion will reduce the effective life of a SCR catalyst.
A combination of a catalytic baghouse and a heat pipe air heater having catalyst coating on its heating surfaces is described in U.S. Pat. No. 4,871,522, granted Oct. 3, 1989 which is assigned to the assignee of the present application and is incorporated herein by reference.
A need remains for an apparatus and method for simultaneous SO.sub.x, NO.sub.x and particulate control which integrates the catalyst inside the fabric filters of the fabric filter house to protect the NO.sub.x reduction catalyst from fly ash erosion and poisoning by sulfur oxides or fly ash. There is also a need for an improved heat recovery system to allow for a lower outlet flue gas temperature to yield an incremental improvement in energy recovery. Further still a need remains for sorbent recycling and reactivation to improve the overall sorbent utilization while decreasing the feed sorbent requirements.